Abstract: In a pilot study it was demonstrated that coronary flow increases immediately following aortic valve replacement. The present study investigates the impact of extracorporeal circulation (ECC), administration of cardioplegia and myocardial ischemia on these changes. In six pigs LAD flow was measured using a perivascular ultrasound Doppler. ECC was established, the ascending aorta was crossclamped and antegrade St. Thomas cristalloid cardioplegia was repeatedly administered. After a period of 60 minutes the aortic crossclamp was released. Following reperfusion for 30 minutes ECC was stopped under stable hemodynamic conditions. LAD flow was then repeatedly measured at normal and high cardiac output (CO). The results were compared to previous findings investigating the impact of mechanical aortic valve replacement on LAD flow in an identical setup. Preoperative coronary flow rates were 23.8"1.3 ml/min. Postoperatively LAD flow increased to 80.3"1.7 ml/min (p<0.001). With rise of CO from 4.2"0.4 l/min to 5.9"0.3 l/min LAD flow was even higher (125.3"9.7 ml/min). This flow rate was not reached by any mechanical aortic valve with the closest results for Medtronic Hall and ADVANTAGE valve substitutes in their optimum orientation. Myocardial ischemia, administration of blood cardioplegia and ECC led to a significant rise of coronary flow. This Acoronary hyperemia@ was not reached by any mechanical aortic valve substitute, but Medtronic Hall and Advantage valves were closer than the St. Jude Medical bileaflet valve.